Current stabilizer circuit for thermionic electron emission device

ABSTRACT

A current stabilizer circuit for a thermionic electron emission device is described in which the cathode heating winding of a power supply transformer is connected to the high-voltage winding through a heating current control circuit. The control circuit senses changes in the emission current of an X-ray tube or other electron emission device having a thermionic cathode, and produces a control signal which is applied to a variable impedance means connected in series with the cathode filament to vary the heating current in order to maintain such emission current substantially constant. The control circuit may operate by series regulation or by pulse duration modulation of the cathode heating current to change the average heating current and temperature of the cathode and thereby vary its electron emission in order to stabilize the emission current in spite of changes in the power supply voltage which tend to vary such emission current.

United States Patent [72] Inventors Peter O. Lauritzen;

Cormack E. Boucher, Seattle, Wash. [2]] Appl. No. 751,927 [22] FiledAug. 12, 1968 [45] Patented Mar. 2, 1971 [73] Assignee AutomationIndustries, Inc.

Los Angeles, Calif.

[54] CURRENT STABILIZER CIRCUIT FOR THERMIONIC ELECTRON EMISSION DEVICE2 Claims, 3 Drawing Figs.

[52] U.S.Cl 315/311, 250/97, 250/103, 307/297, 315/302 [51] Int. Cl.1105g H34 [50] Field of Search 250/97, 99, 103; 307/297; 315/291, 302,311

[5 6] References Cited UNITED STATES PATENTS 2,617,045 11/1952 Coe250/97 2,810,838 10/1957 Clappetal 3,327,131 6/1967 Grimmer ABSTRACT: Acurrent stabilizer circuit for a thermionic electron emission device isdescribed in which the cathode heating winding of a power supplytransformer is connected to the high-voltage winding through a heatingcurrent control circuit. The control circuit senses changes in theemission current of an X-ray tube or other electron emission devicehaving a thermionic cathode, and produces a control signal which isapplied to a variable impedance means connected in series with thecathode filament to vary the heating current in order to maintain suchemission current substantially constant. The control circuit may operateby series regulation or by pulse duration modulation of the cathodeheating current to change the average heating current and temperature ofthe cathode and thereby vary its electron emission in order to stabilizethe emission current in spite of changes in the power supply voltagewhich tend to vary such emission current.

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VOLTAGE V AT POINT Bl VOLTAGE v AT POINT s PETER O. LAURlTZEb CORMACK E.BOUCHEF INVENTORS BUCKHORN, BLORE, KLAROU/S 7' 8 SPAR/(MAN ATTORNEYSPATENTEDMAR 2mm 3.667.995

SHEET 2 BF 2 PETER O. LAURITZEN CORMACK E. BOUCHER INVENTORS. BY

BUG/(HORN, BLORE, KLAROU/ST a SPAR/(MAN ATTORNEYS CURRENT STABILIZERCIRCUIT FOR THERMIONIC ELECTRON EMISSION DEVICE BACKGROUND OF THEINVENTION The subject matter of the present invention relates generallyto the stabilization of emission current in an electron emission deviceemploying a thermionic cathode to maintain such emission currentsubstantially constant, and in particular to an improved stabilizercircuit which compensates for changes in any supply voltage appliedacross such device which otherwise would tend to vary the emissioncurrent. More specifically, the present current stabilizer circuitincludes a control circuit connected between the high-voltage cathodewinding and the cathode heating winding of a power supply transformer.The control circuit senses changes in emission current and derives acontrol signal therefrom which is applied to a transistor or othervariable-impedance element connected in series with the cathode filamentto vary the heating current in an opposite manner to the changes inemission current. In one embodiment the control circuit operates byseries regulation of the heating current, while in another embodiment itoperates by pulse duration modulation of such heating current.

The current stabilizer circuit of the present invention may be employedwith an X-ray tube having a filament cathode but may also be employedwith other electron emission devices such as electron beam welders,cathode ray tubes, electron microscopes which employ thermionic cathodesand are operated in a temperature limited thermionic emissionmode sothat the cathode temperature controls the amount of anode-to-cathodeemission current. i

The current stabilizer circuit of the present invention has severaladvantages over previous stabilizer circuits such as that shown in US.Pat. No. 2,617,045 of M. R. Coe and US. Pat. No. 2,627,035 of J. Ballwhich employ separate heater transformers to supply heating current tothe cathode filament of the X-ray tube. These heater transformers arequite bulky and expensive since they must be insulated for the highvoltage applied to the cathode of the X-ray tube. The present circuitavoids the need for such a highly insulated filament transformer byproviding the heating winding as a secondary winding on the sametransformer core as the high-voltage cathode winding and connecting thecontrol circuit between such high voltage winding and such heatingwinding.

The stabilizer circuit of the present invention has the advantage ofproviding an electron emission apparatus of smaller size, light weight,whose emission current is maintained substantially constant over a widerange of anode-to-cathode voltage adjustments. The present circuit issimpler and more reliable than previous current stabilizer circuits andoperates in a fast, accurate manner. Also the pulse duration modulationembodiment of the present circuit has the additional advantage ofdissipating very little power since the transistors in the controlcircuit operate as switches. As a result no special provisions for heatremoval are needed, which would be difficult since the circuit is notconnected to ground potential except through the high-voltage secondarywinding of the power supply transformer so that any heat conduction pathto the chassis or other grounded heat sink would have to be highlyinsulated to prevent current leakage. This connection has the advantagethat the current stabilizer circuit and the high-voltage cathode windingcan be contained within the same insulated housing with no externalconnections, to provide the small size and light weight.

It is therefore one object of the present invention to provide animproved current stabilizer circuit for an electron emission devicehaving a thermionic cathode, which maintains the emission current ofsuch device substantially constant in spite of variations in powersupply voltage.

Another object of the present invention is to provide an improvedcurrent stabilizer circuit which is of small size and light weight andwhich operates in a simple, reliable and accurate manner.

Still another object of the invention is to provide an improved X-rayapparatus including a current stabilizer circuit which is connected toground only through the high-voltage secondary winding of a power supplytransformer and whose supply potential is added to the high voltage onsuch winding to enable such circuit to be contained within the sameinsulated housing as the high-voltage secondary winding of suchtransformer with no external connections.

A further object of the invention is to provide an improved currentstabilizer circuit of low power dissipation which employs pulse durationmodulation to vary the cathode heating current of an electron emissiondevice in order to maintain the emission current of such devicesubstantially constant in spite of voltage variations at the powersupply transformer.

An additional object of the present invention is to provide an improvedcurrent stabilizer circuit for an X-ray apparatus in which a controlcircuit is employed between the cathode heating transformer winding andthe high-voltage cathode winding of the same power supply transformer inorder to maintain the emission current of the X-ray tube substantiallyconstant while enabling the anode-to-cathode voltage of the X-ray tubeto be varied over a wide range.

BRIEF DESCRIPTION OF DRAWINGS Other objects and advantages of thepresent invention will be apparent from the following detaileddescription of preferred embodiments thereof and from the attacheddrawings of which:

FIG. 1 is a schematic diagram of a current stabilizer circuit inaccordance with the present invention which can be operated either in aseries regulation mode or a pulse duration modulation mode;

FIG. 2 is a detailed circuit diagram of the preferred embodiment of thecurrent stabilizer circuit of the present invention which operates bypulse duration modulation; and

FIG. 3 shows the waveforms of electrical signals produced during theoperation of the circuit of FIG. 2.

DESCRIPTION OF PREFERRED EMBODIMENTS As shown in FIG. 1, an X-ray tube10 or other electron emission device, has its anode 12 and filamentcathode 14'- connected to high-voltage transformers I6 and 18,respectively, which provide an AC power supply for such tube of highpeakvoltage. Of course the power supply can also be a single high-voltagetransformer having a grounded center tap on its secondary winding forsupplying both the positive and negative high voltage at the oppositeends of such winding. ln order to vary the anode-to-cathode voltageacross the X-ray tube 10, a variable auto transformer 19 may be providedin the primary winding circuits of the high voltage transformers l6 and18 whose primary windings 20 and 22, respectively are connected inparallel across an AC power supply 24, which may be conventional volts60 c.p.s. AC powerline. The anode 12 of the X-ray tube is connected toone terminal of a highvoltage secondary winding 26 on transformer 16,while the cathode 14 is connected through the control circuit of thepresent invention to one terminal of a high voltage secondary winding 28on transformer 18. The other terminals of the secondary windings 26 and28 are grounded. The polarity of the secondary windings 26 and 28 issuch that for one-half cycle a positive voltage is applied to the anodeand simultaneously a negative voltage is applied to the cathode toprovide a total peak voltage of, for example, approximately kilovoltsacross the X-ray tube, which causes a current to flow between thecathode and anode of such tube of about 2 to 3 milliamperes.

Another secondary winding 30 forming the heating winding is provided onthe high-voltage end of transformer 19 with its opposite ends connectedtogether through rectifiers 32 and 34 and their common anode terminal isconnected to one terminal of the cathode filament 14, while the otherterminal of such filament is connected to a centertap on such heatingwinding. When the variable auto transformer 19 is adjusted to change thevoltage applied between the anode and cathode of the X-ray tube, itcauses a corresponding change in heating voltage across secondarywinding 30. This tends to vary the heating current and the electronemission of the cathode which causes an undesirable change in emissioncurrent of the X-ray tube. The current stabilizer circuit of the presentinvention maintains this emission current substantially constant inspite of changes in power supply voltage due to adjustment of autotransformer 19 and other factors such as changes in emissioncharacteristics of the filament caused by aging of the X- ray tube ortube replacement. It should be noted that while the X-ray tube shown isa diode, it may be a triode or any other tube which operates withtemperature limited thermionic emission, rather than spacecharge-limited emission, so that the filament temperature alone controlsthe amount of emission current. This enables variations in heatingcurrent to cause corresponding changes in the emission current.

The current stabilizer circuit of the present invention includes asignal controlled variable impedance connected in series with filament14. The variable impedance may be in the form of a transistor 36 of theNPNv type having its collector connected to one terminal of the cathodefilament 14 at point B and its emitter connected to the anodes ofrectiflers32 and 34 at point A. The base of transistor 36 is connectedto the output of a control amplifier 38 whose input is connected througha coupling resistor 40 to the "high-voltage terminalof transformerwinding 28. A source of substantially constant current 42 is alsoconnected to the same high-voltage terminal of transformer winding 28 ata current summing point 44 to provide a current comparator means ashereafter discussed. Current source 42 provides a DC reference current(I of predetermined value from which the high-voltage current (I cathodewinding 28 is subtracted to produce a correction current (1 which isapplied as a control voltage input signal to the input of amplifier 38.The amplifier 38 applies the control signal corresponding to suchcorrection current, to the base of the transistor 36 in order to varythe impedanceof such transistor and change the heating current flowingthrough filament 14. Such a control signal is produced when the current(1, in the high-voltage secondary winding 28 corresponding to the X-raytube emission current (I differs from that of the reference current (Isupplied by current source 42.

The DC power supply for the amplifier 38 and the current source 42 isprovided by a rectifier and voltage regulator circuit 46, having itsinput connected across a third secondary winding 48 provided ontransformer 18 at the high-voltage end of cathode winding 28. Theregulated DC output voltage of the regulator circuit 46 is applied ,tothe constant current source 42 and to the common terminal of the cathodefilament Find the center tap of the heating winding 30. This point C atthe output of the voltage regulator circuit 46 is effectively the signalground of the circuit.

A shunt capacitor 50 is connected between the output of "the voltageregulator 46 and the coupling resistor 40 so that it forms a filtercircuit with such coupling resistor for smoothing or averaging thecorrection 'current (I to provide a slowly v varying DC control currentat the input of amplifier 38. This control current provides an inputlevel for the amplifier which changes by an amount corresponding to thedifference between high-voltage winding current (I and the referencecurrent (I The output signal of the amplifier38 which is employed as thecontrol signal applied to transistor 36,'may be the slowly varying DCsignal produced on capacitor 50 to provide series regulation of theheating current or it may be the form of pulses whose width varieswiththe value of such slowly varying DC signal to provide pulse durationmodulation (PDM) of such heating current, depending upon the position ofa switch 52. The switch 52 is a two position switch which in the PDMposition shown connects point A to the input of the control amplifier 38through an AC coupling network including a rechanges the heating currentflowing through filament 14 in a manner opposite to the changes in thecurrent flowing through secondary winding 28 corresponding to theanode-to-cathode emission current of the X-ray tube 10. Thus, decreasesin emission current (I cause a decrease in the current (I in the highvoltage winding 28 under the reference current (I which producesincrease in correction current (I that is transmitted through the filternetwork 40 and 50 to produce a slowly varying DC control signal. Thiscontrol signal is applied to the base of transistor 36 at the output ofthe control amplifier rendering such transistor more conducting andincreases the heating current flowing in filament l4. This increase inheating current raises the filament temperature which in turn increasesthe anode-to-cathode emission current in the tube until the current (1flowing in secondary winding 28 again becomes substantially equal to thereference current (I provided by current source 42, so that thecorrection current decreases to a small value, thereby stopping furtherdecreases in the heating current. As a result the emission currentreturns to its previous value, which is set'at a predetermined currentby adjustment of the value of the reference current within thecurrentsource 42. It should be noted that the forward bias voltage forthe control amplifier 38 is provided by the small amount of correctioncurrent (I which normally flows to charge the filter capacitor 50 tosuch bias voltage.

In the PDM position of the switch 52 in the circuit of FIG. 1, thetransistors in the control amplifier 38 and the variable impedancetransistor 36 are operated as switches, with transistor 36 beingquiescently biased ON or conducting. In FIG. 3 nega tive half cycles 58of the full wave rectified sine wave output signal of the heatingcurrent supplied through diodes 32 and 34 to point A are shown withrespect to the signal ground point C, as are the other signals in thisFIG. The AC signal 58 is transmitted through the AC coupling networkincluding elements 54 and 56 to the input of control amplifier 38. TheAC signal 58 flowing through the AC coupling elements 54 and 56periodically switches control amplifier 38 OFF to render transistor 36nonconducting and produce heating voltage pulses 60 or 62 at point Bwhich terminate when signal 58 crosses switching levels'64 or 66. Thisswitching OFF occurs when the sum of the current through resistor 54 andthe current through resistor 40 causes the direction of current flow atthe input of the amplifier 38 to be away from the amplifier. When thesum of the currents through resistors 54 and 40 causes the net currentto flow towards the amplifier input38 the transistor 36 is switched ONand rendered conducting to apply the instan taneous value of voltage 58across the filament 14. The particular switching level 66 or 64 isdetermined by the control voltage signal produced on capacitor 50 by thecorrection current (I transmitted from the summing point 44. As a resultheating voltage pulses 62 of narrower width Y are produced at point Bwhen the control current through 40 is reduced and generates theswitching level 66. Similarly heating pulses 60 of greater width X areproduced when the control current through 40 is relatively largercausing the switching level to move to 64. This pulse durationmodulation operation varies the average heating current of the filamentwhich changes the temperature and electron emission of such filament tomaintain the emission current constant. Pulse duration modulation hasthe added advantage that the control amplifier and transistor 36dissipate less power, since they are operated as switches.

A detailed circuit diagram of a current stabilizer circuit employingsuch pulse duration modulation is shown in FIG. 2, in which the samereference numerals have been employed to designate elements similar tothose in FIG. 1 and for this reason such elements will not be discussed.The control amplitier 38 includes an input transistor 68 of NPN type,having its base connected to resistor 40, its emitter connected to theanode of a Zener diode 70 in the voltage regulator 46, and its collectorconnected through a coupling resistor 72 to the base of a secondtransistor 74 of PNP type. The emitter of transistor 74 is connectedthrough a resistor 76 to the cathode of another Zener diode 78 in thevoltage regulator 46, while its collector is connected through a loadresistor 80 to the base of the variable impedance transistor 36. Inaddition. a third transistor 82 of PNP type is provided in the controlamplifier 38 with its base connected to the emitter of transistor 74,its emitter connected to a signal ground conductor 84 which con nectsthe center tap of the heating winding 30 to the common terminal of thecathode of diode 70 and the anode of diode 78. The collector oftransistor 82 is connected through a load resistor 86 to the base of thetransistor 36. p

A normally nonconducting Zener diode 87 may be connected across theemitter to collector circuit of transistor 36 to provide overvoltageprotection for such transistor and a five ampere fuse 88 may beconnected in series with a filament 14 to protect such filament againsttoo high a heating current. A frequency compensation circuit including adiode 90 in series with a resistor 92 may be connected between theemitter of transistor 68 and the common terminal of resistor 40 andcapacitor 50.

The rectifier and voltage regulator circuit 46 includes a full waverectifier bridge formed by four diodes 94, 96, 98 and 100. The anodes ofdiodes 94 and 98 and the cathodes of diodes 96 and 100 are respectivelyconnected to the opposite terminals of a filter capacitor 102. The Zenerdiodes 70 and 78 form part of the voltage regulator circuit with thecathode of diode 78 connected through a coupling resistor 106 to theupper terminal of capacitor 102 and the anode of diode 70 connected tothe lower terminal of such capacitor. Capacitor 102 is charged to a DCvoltage which is sufficient to cause reverse breakdown of the Zenerdiodes 70 and 78. As a result a substantially constant voltage of about10.7 volts is produced across the Zener diodes 70 and 78 and applied asthe supply voltage of the control amplifier 38. Of course the absolutevalues of voltage in the entire circuit including that on the Zenerdiodes vary with the AC voltage on the high-voltage secondary winding 28to which they are connected. A coupling resistor 104 is connectedbetween the upper terminal of such filter capacitor and the cathodes ofdiodes 96 and 100.

The common connection at the anode of Zener diode 78 and the cathode ofZener diodes 70 is connected to the base of a transistor 108 of PNPtype, forming the constant current source 42. The collector oftransistor 108 is connected to the current summing point 44 and itsemitter is connected through a variable resistor 110 to the cathode ofZener diode 78. The Zener diode 78 has a reverse breakdown voltage of5.1 volts and is normally conducting to provide this voltage across theseries circuit including the variable resistor 110 and theemitter-to-base junction of transistor 108, which maintains theemitter-to-collector current of such transistor substantially constant.The reference current (I flowing in the collector of transistor 108 maybe varied by adjusting variable resistor 110. Thus the setting of thisresistor determines the reference current and indirectly the emissioncurrent of the X-ray tube. It should be noted that the emission currentof the X-ray is on the order of 2 to 3 milliamperes, while the heatingcurrent flowing through transistor 36 is ordinarily about 4 amperes RMS,with ampere peak values. Therefore transistor 36 through which theheating current flows must be a high-power transistor, but the othertransistors 68, 74, 82 and 108 of the control circuit may be ofconventional type.

Typical values of the components employed in the circuit of FIG. 2 areas follows:

transistor 36 type 2N377l transistor 68 type 2N2473 transistors 74, 82type 2N4030 transistor 108 type 2N3962 resistors 40, 72, 92 2.2 k!)capacitor 50 50 pf.

resistor 76 10 k!) 47 k 200 k [2 k!) l uf 5 kQ(variableI 100 k0.

resistor 54 resistor resistor 86 capacitor 56 resistor ll0 resistor [04resistor I06 2 KO.

capacitor 102 15 uf.

diode type lN400l diode 87 type 1N3024 diodes 32 and 34 type MR1 l2]diode 78 5. 1 volts Zener diode 70 5.6 volts Zener.

It will be obvious to those having ordinary skill in the art that manychanges may be made in the details of the above described preferredembodiment of the present invention without departing from the spirit ofthe invention. For example, pulse duration modulation can also beachieved through the filter circuit 40 and 50 by causing such filtercircuit to be less efficient in smoothing the correction signal, sincethe X- ray tube functions as a rectifier to provide half way rectifiedpulses of correction voltage which can be used in a similar manner tothe negative pulses transmitted through the AC coupling networkincluding resistor 54 and capacitor 56, thereby eliminating the need forthese components. Therefore the scope of the present invention shouldonly be determined by the following claims.

We claim: 1. A current stabilizer circuit for an electron emissiondevice having a thermionic cathode, comprising:

transformer means including a high-voltage winding having one endconnected for applying a high voltage to said cathode to cause emissioncurrent to flow in said device, and a heating winding for supplyingheating current to said cathode, said heating winding being provided asa secondary winding on the same transformer core as said high-voltagewinding; control circuit means connected between said one end of saidhigh-voltage winding and said heating winding for producing a controlsignal proportional to variations of current in the high-voltage windingcorresponding to changes in said emission current; variable-impedancemeans connected to said heating winding and said control circuit meansfor varying said heating current in response to said control signal,said variableimpedance means causing the heating current to vary in anopposite manner to the changes in said emission current to compensatefor said changes in order to stabilize said emission current andmaintain it substantially constant; said control circuit means includinga source of substantially constant reference current, comparator meansfor subtracting said reference current and the current in said highvoltage winding to produce a difi'erence current from which the controlmeans derives the control signal, an amplifier having its inputconnected to the output of the comparator means, and filter meansincluding a shunt capacitor connected across the input of said amplifierfor changing the AC difference current to a slowly varying DC inputlevel; and said amplifier also being connected at its input to theheating winding through an AC coupling capacitor and rectifier means toapply pulses of one polarity to the input of said amplifier which switchthe amplifier between conduction and nonconduction when the amplitude ofsaid pulses exceeds the DC input level and cause said amplifier toproduce corresponding output pulses whose widths are modulated bychanges in the value of said DC input level to provide said controlsignal.

2. A circuit in accordance with claim 1 in which the pulse durationmodulated control signal is applied to the base electrode of atransistor having its emitter-to-collector circuit in series with theheating winding, for switching such transistor on and off to vary theamount of average heating current flowing through the cathode filament.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 1 r995 Dat d March 2 1971 Peter O. Lauritzen and Cormack E. BoucherInventor(s) It is certified that error appears in the above-identifiedpatent and that said Letters Patent are hereby corrected as shown below:

Column 3, line 35, after (I should be --flI in through the highvoltage-;

Column 3, line 69, after "may be" should be --i:

Column 5, line 61, after "X-ray" should be --tu1 Signed and sealed this20th day of July 1971.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. WILLIAM E. SCHUYLER, JR. Attesting OfficerCommissioner of Patents

1. A current stabilizer circuit for an electron emission device having athermionic cathode, comprising: transformer means including ahigh-voltage winding having one end connected for applying a highvoltage to said cathode to cause emission current to flow in saiddevice, and a heating winding for supplying heating current to saidcathode, said heating winding being provided as a secondary winding onthe same transformer core as said high-voltage winding; control circuitmeans connected between said one end of said high-voltage winding andsaid heating winding for producing a control signal proportional tovariations of current in the high-voltage winding corresponding tochanges in said emission current; variable-impedance means connected tosaid heating winding and said control circuit means for varying saidheating current in response to said control signal, saidvariable-impedance means causing the heating current to vary in anopposite manner to the changes in said emission current to compensatefor said changes in order to stabilize said emission current andmaintain it substantially constant; said control circuit means includinga source of substantially constant reference current, comparator meansfor subtracting said reference current and the current in said highvoltage winding to produce a difference current from which the controlmeans derives the control signal, an amplifier having its inputconnected to the output of the comparator means, and filter meansincluding a shunt capacitor connected across the input of said amplifierfor changing the AC difference current to a slowly varying DC inputlevel; and said amplifier also being connected at its input to theheating winding through an AC coupling capacitor and rectifier means toapply pulses of one polarity to the input of said amplifier which switchthe amplifier between conduction and nonconduction when the amplitude ofsaid pulses exceeds the DC input level and cause said amplifier toproduce corresponding output pulses whose widths are modulated bychanges in the value of said DC input level to provide said controlsignal.
 2. A circuit in accordance with claim 1 in which the pulseduration modulated control signal is applied to the base electrode of atransistor having its emitter-to-collector circuit in series with theheating winding, for switching such transistor on and off to vary theamount of average heating current flowing through the cathode filament.